Apparatus and method of determining an endpoint during a chemical-mechanical polishing process

ABSTRACT

The present invention provides a polishing apparatus for use in polishing a substrate, including: (1) A polishing platen, and (2) a rotational strain sensor coupled to the polishing platen configured to detect a change in a rotational strain of the polishing platen during a polishing process. In addition, the present invention provides an accompanying method of detecting an endpoint during the polishing process by detecting a change between a first rotational strain and a second rotational strain with the rotational strain sensor.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is directed, in general, to chemical-mechanical polishing (CMP) of a semiconductor wafer and, more specifically, to a method of determining an endpoint by monitoring rotational strain during the CMP process and a polishing apparatus including the same.

BACKGROUND OF THE INVENTION

[0002] In the fabrication of semiconductor components, metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits (ICs). The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, Chemical Vapor Deposition (CVD) of oxide or application of Spin On Glass (SOG) layers followed by fellow processes. Holes, or vias, formed through the insulating layers provide electrical connectivity between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces.

[0003] Also, deep (greater than 3 μm) and narrow (less than 2 μm) trench structures have been used in advanced semiconductor design for three major purposes: (1) to prevent latch-up and to isolate n-channel from p-channel devices in CMOS circuits; (2) to isolate the transistors of bipolar circuits; and (3) to serve as storage-capacitor structures in DRAMS. However, in this technology it is even more crucial to precisely determine the endpoint of differing materials to prevent unnecessary dishing out of the connector metal.

[0004] Chemical-mechanical polishing (CMP) has been developed for providing smooth insulator topographies. Briefly, the CMP processes involve holding and rotating a thin, reasonably flat semiconductor wafer against a wetted polishing surface under controlled chemical, pressure, and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals which etch or oxidize various surfaces of the wafer during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarity of the polished surface.

[0005] CMP is also used to remove different layers of material from the surface of a semiconductor wafer. For example, following via formation in a dielectric material layer, a metallization layer is blanket-deposited, and then CMP is used to produce planar metal studs. When used for this purpose, it is important to remove a sufficient amount of material to provide a smooth surface, without removing an excessive amount of underlying materials. The accurate removal of material is particularly important in today's submicron technologies where the layers between device and metal levels are constantly getting thinner. To better determine endpoints between removed and remaining layers of a semiconductor wafer, an accurate polishing endpoint detection technique is invaluable.

[0006] In the past, endpoints have been detected by interrupting the CMP process, removing the wafer from the polishing apparatus, and physically examining the wafer surface by techniques which ascertain film thickness and/or surface topography. However, with such prior art processes if the wafer did not meet specifications, it was loaded back into the polishing apparatus for further polishing to achieve the desired planarity. This would have to be repeated until a sufficient amount of material was removed. Unfortunately, in addition to the excess time required by this technique, if too much material was removed, the wafer was likely found to be substandard to the required specifications, and often discarded altogether. By experience, an elapsed CMP time for a given CMP process has been developed with some accuracy. However, like the prior art technique described above, this endpoint detection technique is time consuming, unreliable, and costly.

[0007] Various active processes have been developed to circumvent the problems associated with prior art endpoint detection techniques. However, these active processes suffer from their own disadvantages and inaccuracies. One of the better known of these prior art techniques involves the continuous monitoring of the motor current of the CMP apparatus. Specifically, the drive motor used to rotate the platen holding the polishing pad is continuously monitored during the polishing process for changes in load current. As each layer of a semiconductor wafer is polished, a certain amount of friction develops between the polishing pad and wafer layer. The amount of friction that develops is dependent, at least in part, on the coefficient of friction present at the interface between the layer being polished and the polishing pad. When the CMP process finishes the removal of one layer of the wafer and begins on the next, a change in the amount of friction between the polishing pad and wafer layer affects the amount of work required by the drive motor. As the work required by the drive motor changes with each different layer, the load current of the motor changes as well. These changes in load current may be monitored to determine when the polishing process has begun on a new wafer layer.

[0008] Unfortunately, the techniques for monitoring the load current of the drive motor also suffer from deficiencies. Specifically, only monitoring changes in the load current of the motor does not obtain layer information directly from the polishing platen. As a result, a change in load current caused by layers composed of materials having similar composition may be too small to detect. Conversely, changes in the load current of the drive motor, such as a power surge, caused by other means may incorrectly inform the operator that an endpoint of a particular layer of the wafer has been reached. With the high cost of semiconductor materials in the industry, a more direct technique for determining a polishing endpoint, with less risk than those found in the prior art, is desirable. Accordingly, what is needed in the art is an improved technique for accurately determining the endpoint of one semiconductor wafer layer and the beginning of the next during a polishing process.

SUMMARY OF THE INVENTION

[0009] To address the above-discussed deficiencies of the prior art, the present invention provides a polishing apparatus for use in polishing a substrate. In one advantageous embodiment, the polishing apparatus provides a polishing platen and a rotational strain sensor coupled to the polishing platen configured to detect a change in a rotational strain of the polishing platen during the polishing process. In addition, the present invention provides a method of detecting an endpoint during polishing of the substrate by detecting a change between a first rotational strain and a second rotational strain with the rotational strain sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0011]FIG. 1 illustrates a polishing apparatus using the motor current detection technique of the prior art;

[0012]FIG. 2A illustrates a polishing pad assembly having a rotational strain detection system according to the principles of the present invention;

[0013]FIG. 2B illustrates a related embodiment of the polishing pad assembly of FIG. 2A;

[0014]FIG. 3A illustrates a polishing pad assembly having an alternative embodiment of the rotational strain detection system of the present invention;

[0015]FIG. 3B illustrates a related embodiment of the polishing pad assembly of FIG. 3A;

[0016]FIG. 4 illustrates a polishing pad assembly having a piezoelectric embodiment of the rotational strain detection system of the present invention;

[0017]FIG. 5 illustrates a polishing pad assembly having yet another embodiment of the detection system of the present invention;

[0018]FIG. 6 illustrates a polishing apparatus using the rotational strain detection system of FIG. 2A;

[0019]FIG. 7A illustrates a close-up view of a first rotational strain detected by the detection system of FIG. 2A;

[0020]FIG. 7B illustrates a close-up view of a second rotational strain detected by the detection system of FIG. 2A; and

[0021]FIG. 8 illustrates a sectional view of a conventional integrated circuit (IC) that may be manufactured according to the principles of the present invention.

DETAILED DESCRIPTION

[0022] Referring initially to FIG. 1, illustrated is a polishing apparatus 100 using the motor current detection technique of the prior art. The polishing apparatus 100 includes a polishing pad 120 for polishing a semiconductor substrate 160 and a platen 110 on which the polishing pad 120 is securely mounted.

[0023] The polishing apparatus 100 further includes a drive motor 130 coupled to a drive shaft 140. The drive shaft 140, in turn, is coupled to the platen 110. During a polishing operation, such as a CMP process, the drive motor 130 is used to turn the drive shaft 140, thereby rotating the platen 110 and polishing pad 120 about a central axis A₁.

[0024] The polishing apparatus 100 still further includes a carrier head 150. Mounted to the carrier head 150 is the substrate 160, such as a semiconductor wafer, that has been selected for polishing. During the polishing process, a downward force 170 is applied to the carrier head 150, causing the carrier head 150 to press the substrate 160 against the polishing pad 120, as the polishing pad 120 is rotated on the platen 110 by the drive motor 130. Optionally, the carrier head 150 may also be rotated during polishing about a second axis A₂.

[0025] In this embodiment of the prior art, the load current of the drive motor 130 used to rotate the platen 110 holding the polishing pad 120 is continuously monitored during the polishing process. As each layer of the substrate 160 is polished, a certain amount of friction develops between the polishing pad 120 and the substrate 160 layer being polished. The amount of friction that develops is dependent, at least in part, on the coefficient of friction present at the interface between the layer being polished and polishing pad 120.

[0026] For example, a dielectric material in a first layer will likely generate less friction than a metal in a second layer. Thus, when the polishing process finishes the removal of the first layer of the substrate 160 and begins polishing the second layer, a change in the amount of friction between the polishing pad 120 and substrate 160 layers affects the amount of work required by the drive motor 130. As the work required by the drive motor 130 changes with each different layer, the load current of the motor 130 changes as well. These changes in load current are monitored by a current meter 180 to determine when the polishing process has begun on a new substrate 160 layer.

[0027] Turning now to FIG. 2A, illustrated is a polishing pad assembly 200 having a rotational strain detection system according to the principles of the present invention. The polishing pad assembly 200 includes a polishing platen 210A with a polishing pad 220 mounted thereon.

[0028] The polishing platen 210A is composed of two pieces, a base plate 260 and an upper plate 270, with the polishing pad 220 fixedly mounted on an upper, outer surface of the upper plate 270. In this embodiment of the present invention, the two plates 260, 270 of the platen 210A are slidably coupled together so that the upper plate 260 will slightly slide against the base plate 270 when a there is an increase in friction between a layer of a substrate (not illustrated) being polished and the polishing pad 220. Alternatively, the upper plate 260 may be composed of a material having more flexibility than the base plate 270. However, in such an embodiment neither plate 260, 270 of the platen 210A is composed of a material so flexible that durability is compromised. Those skilled in the art understand that even extremely hard materials still maintain a certain amount of flexibility. As such, the upper plate 260 need only be composed of a material having some increased flexibility over the material of the base plate 270, even if only a slight amount. Moreover, the present invention is not limited to any particular means by which to couple the upper 260 and base 270 plates.

[0029] The polishing pad assembly 200 further includes a drive shaft 250 coupled to the base plate 270 of the polishing platen 210A. As with the polishing apparatus 100 of FIG. 1, the drive shaft 250 is rotated by a drive motor (not illustrated) about a central axis A₁ during a polishing process. This, in turn, also rotates the platen 210A and the polishing pad 220 about the central axis A₁.

[0030] In this particularly advantageous embodiment, the rotational strain detection system of the present invention includes a backing block 240 projecting from the base plate 270 of the platen 210A. The backing block 240 projects towards the polishing pad 220 and into a cavity 280 in the upper plate 260 configured to receive the backing block 240. In addition, the polishing pad assembly 200 includes a rotational strain sensor 230. The rotational strain sensor 230 is located between an outer face of the backing block 240 and a face of the cavity 280 in the upper plate 260. These faces cooperate to compress the rotational strain sensor 230 to determine the endpoint between layers of the substrate being polishing.

[0031] Specifically, during a polishing process the platen 210A and polishing pad 220 are rotated at a preset velocity by the drive shaft 250. A substrate selected for polishing is then pressed downward, against the polishing pad 220. Since the polishing surface of the polishing pad 220 has a predetermined texture, a friction force develops between the layer being polished and the polishing pad 220. While overcoming this friction and maintaining rotation of the platen 210A at the original velocity, the interface between the upper plate 260 and the base plate 270 experiences some degree of strain just before the upper plate 260 begins to slide against the base plate 270. However, since the upper plate 260 does not yet begin to slide against the base plate 270, the strain is not enough to trigger the rotational strain sensor 230. More specifically, the strain occurs as the shaft 250 attempts to continue the platen's 210A rotation at the original velocity while the friction between the polishing pad 220 and the first layer of the substrate works to slow or stop the upper plate's 260 rotation.

[0032] Then, when the first layer of the substrate has been sufficiently removed, and polishing begins on a second layer having a higher coefficient of friction, the rotational strain experienced by the two plates 260, 270 of the platen 210A changes. Specifically, as the drive shaft 250 continues to rotate the platen 210A at the original velocity and the friction between the second layer of the substrate and the polishing pad 220 increases, the platen 210A undergoes an increase in rotational strain. Since the rotational strain sensor 230 is positioned between the backing block 240 affixed to the base plate 270 and the wall of the cavity 280 in the upper plate 260, the increase in rotational strain causes the upper plate 260 to momentarily slow its rotation compared to the base plate 270. This, in turn causes the upper plate 260 to slightly slide against the base plate 270, and the cavity wall to compress the sensor 230 against the backing block 240. This compression triggers the rotational strain sensor 230, indicating that the endpoint of the first layer and the beginning of the second layer has been reached.

[0033] Those skilled in the art will understand that any number of sensors may be used for the rotational strain sensor 230. For example, a sensor simply indicating the difference between two degrees of rotational strain may be employed in the assembly 200. However, a sensor having multiple trigger points, configured to send different signals in response to various increases or decreases in friction between the polishing pad 220 and multiple layers of the substrate may also be employed. Thus, in its broadest form, a rotational strain detection system according to the present invention encompasses a number of different types of sensors capable of detecting a broad range of rotational strains for use as a rotational strain sensor 230.

[0034] Moreover, the rotational strain sensor 230 is not limited to detecting an increase in frictional forces. For example, the rotational strain sensor 230 may be compressed during the polishing of the first layer of substrate, and then signal a decrease in friction when polishing of a second layer, having a lower coefficient of friction, begins. In such an embodiment, it is not the compression of the rotational strain sensor 230 between the cavity 280 wall when the upper plate 260 slightly slides against the backing block 240 of the base plate 270 that indicates an endpoint has been reached, but rather the lack of compression that indicates the endpoint. As before, in this embodiment the rotational strain sensor 230 may be configured to detect multiple changes in friction between the substrate layers and the polishing pad 220.

[0035] Turning briefly to FIG. 2B, illustrated is a related embodiment of the polishing pad assembly 200 of FIG. 2A. Specifically, FIG. 2B illustrates a polishing platen 210B having only a single plate. In this embodiment, the backing block 240 extends from the platen 210B and is received by a cavity 280 formed in the polishing pad 220 rather than in an upper plate. The rotational strain sensor 230 is still located between an outer face of the backing block 240 and the wall of the cavity 280.

[0036] To determined the endpoint between two substrate layers in this embodiment, the rotational strain sensor 230 is still compressed between the cavity 280 wall and the backing block 240 as the rotational strain of the platen 210B is increased. As illustrated, the only difference is that the cavity 280 wall is now part of the polishing pad 220 rather than an upper plate of the polishing platen 210B. As a result, the increase in rotational strain evidenced by the polishing pad 220 is transmitted to the rotational strain sensor 230, as described with respect to FIG. 2A, indicating an endpoint between the first and second layers. Of course, as with the previous embodiments discussed above, the strain sensor 230 may alternatively be triggered by a decrease in rotational strain rather than an increase, and may be configured to detect various changes in friction caused by multiple substrate layers.

[0037] By monitoring and detecting changes directly from the source of those changes, rather than merely monitoring the drive motor or other components of a polishing apparatus, more accurate and precise information regarding the polishing process may be obtained. Those skilled in the art understand that reliance on indirect information may be costly if that information is unknowingly altered or otherwise corrupted by factors external to the critical item or items being monitored. In short, using information gathered directly from the source, such as the contact point between a semiconductor substrate and a polishing pad, allows the operator or other authorized personnel to proceed with the polishing operation, confident that the endpoints of the various layers of the substrate are accurately determined throughout the entire polishing process. As discussed above, such accurate determination of endpoints substantially reduces the risk of damage to the substrate undergoing the polishing process.

[0038] Referring now concurrently to FIGS. 3A and 3B, illustrated is a polishing pad assembly 300 having an alternative embodiment of the rotational strain detection system of the present invention. As before, the polishing pad assembly 300 includes a polishing platen 310A with a polishing pad 320 mounted thereon. FIG. 3A illustrates the platen 310A as having an upper plate 360 and a base plate 370, while FIG. 3B illustrates the platen 310B as a single plate. As before, the two plates 360, 370 of the platen 310A are slidably coupled together so that the upper plate 360 will slightly slide against the base plate 370 when a there is an increase in friction between a layer of a substrate (not illustrated) being polished and the polishing pad 320. Alternatively, the upper plate 360 may be composed of a material having more flexibility than the base plate 370.

[0039] The polishing pad assembly 300 includes a rotational strain sensor 330 having a stud member 340 extending therefrom. In FIG. 3A, the rotational strain sensor 330 is located in the base plate 370 of the polishing platen 310A. Additionally, the stud member 340 extends from the base plate 370 of the platen 310 and is received by a cavity 380 formed in the upper plate 360 of the platen 310A. In FIG. 3B, the rotational strain sensor 330 is located in the platen 310B, but is positioned nearest the polishing pad 320. Therefore, the stud member 340 extends from the polishing platen 310B and is received by a cavity 380 formed in the polishing pad 320 rather than remaining entirely in the platen 310B.

[0040] As discussed above, as the endpoint of a first layer of substrate is reached a change in rotational strain occurs when polishing of the second layer begins. In FIG. 3A, as a drive shaft 350 attempts to maintain the platen's 310A rotational velocity, an increase (or decrease) in rotational strain will occur in the upper and base plates 360, 370 of the polishing platen 310A, based on the friction between the second layer and the polishing pad 320. The change in friction will cause the upper plate 360 to slightly slide against the base plate 370, with the direction of the slight slide depending on whether friction is increasing or decreasing. In FIG. 3B, the change in rotational strain occurs primarily in the polishing pad 320, which is comprised of a more flexible material than the platen 310B. The rotational strain caused by the change in friction forces the polishing pad 320 to slightly distort, twisting slightly at the middle while its base is firmly adhered to the platen 310B.

[0041] In either embodiment, however, the stud member 340 and rotational strain sensor 330 react in a similar manner. In FIG. 3A, the wall of the cavity 380 formed in the upper plate 360 of the platen 310A eventually contacts and moves the stud member 340 as the rotational strain increases. Alternatively, the stud member 340 has already been contacted and is no longer contacted as the rotational strain decreases. Similarly, in FIG. 3B the cavity 380, thus it is the polishing pad 320 that eventually contacts and moves the stud member 340 in response to an increase in rotational strain, or removes any contact from the stud member 340 in response to a decrease in rotational strain.

[0042] In one embodiment of the present invention, the stud member 340 is a flexible stud member 340 having electrical conductors placed within the body of the member 340. In this embodiment, the conductors are adjacent, but spaced-apart so as not to contact one another until the flexible member 340 is bent. In an alternative embodiment, the stud member 340 is rigid but affixed to a flexible base, designated 390 in FIG. 3B, located in the rotational strain sensor 330. The flexible base 390 would then include the adjacent conductors, spaced so as not to contact one another until the entire rigid stud member 340 is tilted so as to flex the base 390. It should be noted that although the flexible base 390 is only illustrated in FIG. 3B, either type of stud member 340 may be employed with the embodiment illustrated in FIGS. 3A and 3B.

[0043] With a flexible stud member 340, when rotational strain increases (or decreases) when an endpoint of a substrate is reached, the wall of the cavity 380 receiving the stud member 340 contacts a side of the stud member 340 and causes it to flex along its shaft. As the shaft of the flexible stud member 340 bends, the conductors within the stud member 340 are also bent so as to contact one another. By contacting, the conductors cause a signal to be sent to the rotational strain sensor 330 to indicate the increase or decrease in rotational strain experienced when a change in friction indicates an endpoint has been reached. The sensor 330 may then transmit a signal to a computer or other type of system (not illustrated) coupled to the sensor 330 to notify an operator of the endpoint. In the embodiment of FIG. 3A, it is the upper plate 360 of the platen 310A that has the cavity 380 and slightly slides against the base plate 370 to bend the flexible stud member 340. In the embodiment of FIG. 3B, the cavity 380 is formed in the polishing pad 320 and it is the polishing pad 320 that slightly twists to contact and bend the stud member 340. Of course, if a reduction in rotational strain is detected, due to a decrease in friction, the upper plate 360 or polishing pad 320 remove pressure from the stud member 340, allowing it to straighten and cause the conductors to cease contacting one another.

[0044] With a rigid stud member 340, when rotational strain increases the wall of the cavity 380 receiving the stud member 340 still contacts a side of the stud member 340. However, in such an embodiment the stud member 340 is rigid and does not flex along its shaft. Instead, the stud member 340 is forced to tilt at the flexible base 390, causing the conductors within the base 390 to bend until they contact one another. As with the flexible stud member 340, when the conductors contact one another a signal is sent to the rotational strain sensor 330 to indicate the increase in rotational strain experienced when an endpoint is reached. The sensor 330 may then transmit a signal to the computer or other system used with the sensor 330. As before, if a reduction in rotational strain is detected, due to a decrease in friction, the upper plate 360 or polishing pad 320 remove pressure from the tilting stud member 340, allowing it to straighten and cause the conductors in the flexible base 390 to cease contacting one another.

[0045] Although two rotational strain sensors 330 are shown in both FIG. 3A and 3B, the present invention is not limited to any particular number of sensors 330 or stud members 340. In its broadest form, the polishing pad assembly 300 can encompass a single sensor 330 or any number of multiple sensors 330. Additionally, a rotational strain detection system using stud members 340 along with rotational strain sensors 330 still provides the same advantages over the prior art described above.

[0046] Looking now at FIG. 4, illustrated is a polishing pad assembly 400 having a piezoelectric embodiment of the rotational strain detection system of the present invention. Those skilled in the art are familiar with the properties of piezoelectric material and its ability to generate an output voltage based on the amount of stress applied to the material.

[0047] In this advantageous embodiment, the polishing pad assembly 400 includes a polishing platen 410 affixed to a drive shaft 450. The drive shaft 450 applies a rotational force to the platen 410, rotating the platen 410 about a central axis A₁. Affixed to an upper surface of the platen 410 is a rotational strain sensor 430 composed of piezoelectric material. Affixed to the opposite face of the piezoelectric rotational strain sensor 430 is a polishing pad 420 for use in polishing layers on a substrate (not illustrated). Since the piezoelectric sensor 430 is mounted on the platen 410, and the polishing pad 420 is mounted to the sensor 430, both the sensor 430 and the pad 420 rotate about the central axis A₁with the polishing platen 410 during the polishing process.

[0048] As the endpoint of one layer of the substrate is reached and polishing begins on a second layer, the rotational strain on the polishing assembly 400 changes in response to the change in friction between the polishing pad 420 and the new layer. As mentioned above, rotational strain is increased in the assembly 400 because the drive shaft 450 attempts to continue the platen's 410 rotation while the friction force between the substrate and the polishing pad 420 attempts to slow or stop that rotation. This increase in rotational strain exerts a stress onto the piezoelectric sensor 430 because it is located between the pad 420 and the platen 410. When this change in stress is applied to the piezoelectric sensor 430, an output voltage 440 of the piezoelectric sensor 430 changes as well. Thus, the change in the output voltage 440 of the sensor 430 may be used to determine when each endpoint of a substrate's layers is reached and polishing begins on a new layer.

[0049] Moreover, because the output voltage 440 of the piezoelectric sensor 430 simply changes as a result of changes in rotational strain, the polishing pad assembly 400 may be used to determine increases or decreases in rotational strain, in accordance with the principles of the present invention. Also, since the output voltage of a piezoelectric material varies across a given range in response to a range of stresses applied to the material, the piezoelectric sensor 430 may be used to determine the endpoints of numerous layers composed of differing materials while still providing the advantages over the prior art discussed above.

[0050] Turning to FIG. 5 there is illustrated a polishing pad assembly 500 having still a further embodiment of the detection system of the present invention. This advantageous embodiment uses a polishing platen 510 and polishing pad 520 combination exactly as found in the prior art. More specifically, the platen 510 is a single plate with an unmodified polishing pad 520 affixed to its upper surface.

[0051] However, in this embodiment a rotational strain sensor 530 is located between a lower surface of the platen 510 and a drive shaft 550 coupled to and used to rotate the platen 510 about a central axis A₁. In the illustrated embodiment, the rotational strain sensor is a torque sensor, and is affixed to the lower surface of the platen 510 and to an upper end of the drive shaft 550 via a collar 540. The rotational strain sensor is coupled between the drive shaft 550 and the platen 510 to determine the amount of torque produced by the drive shaft 550 when turning the platen 510 during various stages of the polishing process. The collar 540 includes a cavity formed therein to house the rotational strain sensor 530, as well as offer structural support between the drive shaft 550 and the polishing platen 510.

[0052] As the drive shaft 550 rotates the platen 510, a change in the friction between the differing substrate layers and the polishing pad 520 causes a corresponding change in rotational strain of the assembly 500, for example the torque between the drive shaft 550 and the platen 510. The drive shaft 550 attempts to continue rotating the platen 510 at the original velocity while the friction acting against the platen's 510 rotation causes a rotational strain to appear at or near the rotational strain sensor 530, i.e., the point where the drive shaft 550 and the platen 510 meet. Thus, the rotational strain sensor 530 detects the changes in rotational strain at this junction point and indicates the changes to a computer or other system (not illustrated) attached thereto. In accordance with present invention, these detected changes indicate to an operator that an endpoint of a substrate layer has been reached and that polishing on a new layer has begun. Of course, the rotational strain sensor 530 illustrated in FIG. 5 is not limited to a torque sensor and may encompass any sensor capable of detecting a rotational strain between the platen 510 and the drive shaft 550.

[0053] Referring now to FIG. 6, illustrated is a polishing apparatus 600 using the rotational strain detection system and polishing pad assembly 200 illustrated in FIG. 2A. The polishing apparatus 600 includes the polishing pad 220 for polishing a semiconductor substrate 630 and the platen 210A on which the polishing pad 220 is securely mounted.

[0054] As discussed with respect to FIG. 2A, the polishing platen 210A includes a base plate 260 and an upper plate 270. In this exemplary embodiment, the base plate 270 is coupled to the drive shaft 250 while the upper plate 260 is used to support the polishing pad 220. As noted before, the upper plate 260 of the platen 210A includes a cavity 280 formed therein and configured to receive the rotational strain sensor 230 and backing block 240 of the rotational strain detection system of the present invention. The backing block 240 is mounted to the base plate 270 of the platen 210A, projecting towards the polishing pad 220 of the polishing apparatus 600 and into the cavity 280.

[0055] The polishing apparatus 600 further includes a drive motor 610 coupled to the drive shaft 250. The drive shaft 250, in turn, is coupled to the base plate 270 and used to rotate the platen 210A, and consequently the polishing pad 220, about a central axis A₁. Further included is a carrier head 620 having the substrate 630, such as a semiconductor wafer that has been selected for polishing, mounted thereon. During the polishing process, a downward force 640 is applied to the carrier head 620, causing the carrier head 620 to press the substrate 630 against the polishing pad 220, while the polishing pad 220 is rotated on the platen 210A by the drive motor 610. Optionally, the carrier head 620 may also be rotated during polishing about a second axis A₂.

[0056] The polishing pad assembly 600 now also includes a computer system 650. The computer system 650 includes signal lines 660 coupled between the rotational strain sensor 230 and a signal input of the computer system 650. As before, as the platen 210A is rotated by the drive motor 610 and a first layer of the substrate 630 is polished, a certain amount of rotational strain is present at the interface between the upper plate 260 and the base plate 270. As discussed above, this first rotational strain is caused by the friction between the first layer of the substrate 630 and the polishing pad 220 working against the rotation upper and base plates 260, 270 of the platen 210A. Then, as an endpoint of the first layer is reached and polishing on a second layer of the substrate 630 begins, an increase or decrease in friction between the second layer and polishing pad 220 directly causes an increase or decrease in the rotational strain, respectively.

[0057] The rotational strain sensor 230 detects this second rotational strain as different than the first rotational strain and signals the computer system 650 via the signal lines 660 that a change in rotational strain has occurred. If an increase in rotational strain is detected the rotational strain sensor 230 is compressed, while if a decrease in rotational strain is detected compression of the rotational strain sensor 230 ceases. Finally, the computer system 650 is programmed to determine whether such a change in rotational strain is indicative of reaching an endpoint for the particular layer being polished. If the computer system 650 has determined that an endpoint has been reached, it can further be programmed to determine which specific endpoint, and thus which particular layer, has been reached by determining the degree of change in rotational strain. Thus, in accordance with the principles of the present invention, the detection system of the polishing apparatus 600 more directly determines the endpoint of a layer being polished by sensing a change in rotational strain based on the friction between the substrate 630 being polished and the polishing pad 220, rather than by merely monitoring the load current of the drive motor 610.

[0058] Turning to FIGS. 7A and 7B, FIG. 7A illustrates a close-up view of a first rotational strain, and FIG. 7B illustrates a close-up view of a second rotational strain, detected by the rotational strain detection system illustrated in FIG. 2A.

[0059] Both FIGS. include the polishing platen 210A and polishing pad 220 of FIG. 2A, with the polishing pad 220 mounted on the platen 210A to perform a polishing operation. Also as before, the polishing platen 210 has an upper plate 260 and a base plate 270. The backing block 240 projects from the base plate, into the cavity 280 in the upper plate 260 configured to receive the backing block 240 and rotational strain sensor 230. The rotational strain sensor 230 is also located in the cavity 280, positioned between a face of the backing block 240 and a wall of the cavity 280.

[0060]FIGS. 7A and 7B further include a substrate 710 having a first layer 720 and a second layer 730. The first layer 720 has a predetermined coefficient of friction for its composition that helps generate a certain amount of friction when polished by the polishing pad 220. This friction, in turn, generates a first rotational strain at the interface between the upper and base plates 260, 270 of the platen 210A. In the illustrated embodiment, the first layer 720 has a relatively low coefficient of friction, such as a dielectric material used to insulate metal components in the substrate 710. In contrast, the second layer 730 has a coefficient of friction higher than that of the first layer 720. In this embodiment, the second layer may be a metal material used in forming transistors or interconnections in the substrate 710. With a higher coefficient of friction, the second layer 730 will help generate a second rotational strain, greater in magnitude than the first rotational strain.

[0061] In FIG. 7A, as the first layer 720 is being polished by the rotating polishing pad 220 the first rotational strain is detected by the rotational strain sensor 230. That information is transmitted to a system (not illustrated) coupled to the sensor 230 for informing an operator when the endpoint of the first layer 720 has been reached. In FIG. 7B, the portion of the first layer 720 being polished in FIG. 7A has been removed. As a result, the second layer 730 is now being polished along with remaining portions of the first layer 720 located at the same depth as the second layer 730. Since polishing the second layer 730 produces more rotational strain, as discussed above, the rotational strain sensor 230 detects the increase in rotational strain when the upper plate 260 slightly slides against the base plate 270, causing the rotational strain sensor 230 to be compressed between the wall of the cavity 280 and the backing block 240. The sensor 230 then transmits this information to the attached system, and the system indicates to an operator that the endpoint of the first layer 720 has been reached and polishing has begun on the second layer 730.

[0062] In an alternative embodiment, the coefficient of friction of the first layer 720 may be greater than the coefficient of friction for the second layer 730. In this embodiment, the resulting first rotational strain is greater than the second rotational strain experienced by the polishing platen 210A. As such, the rotational strain sensor 230 detects a decrease in rotational strain rather than an increase. Of course, in such an embodiment the system to which the strain sensor 230 is attached would be programmed to determine that the endpoint of the first layer 720 has been reached by the decrease in rotational strain. In yet another embodiment of the present invention, the system would be programmed with information regarding degrees of increase and decrease in rotational strain, caused by corresponding increases and decreases in friction between the polishing pad 220 and substrate 630, in order to determine any of a number of corresponding endpoints reached throughout the polishing process. In such an embodiment, the operator of the polishing apparatus would be informed when the endpoint of each layer of the substrate 630 was reached, regardless of the composition of that layer.

[0063] Those skilled in the art are familiar with the advantages of employing sensors having high precision and accuracy. In addition, those advantages are further increased when using sensors that provide a high degree of sensitivity. In sum, employing an accurate and precise sensor, with appreciable sensitivity, as a rotational strain sensor in accordance with the principles of the present invention results in a direct detection system that assists a polishing apparatus in providing superior polishing for a semiconductor substrate. Moreover, such superior polishing is achieved while still protecting against over-polishing the layers of a substrate and overcoming deficiencies associated with the techniques found in the prior art.

[0064] Turning finally to FIG. 8, illustrated is a sectional view of a conventional integrated circuit (IC) 800 that may be manufactured according to the principles of the present invention. The IC may be derived from the edge portion of a wafer after completing a CMP process with the rotational strain detection system of the present invention.

[0065] The integrated circuit 800 may include a CMOS device, a BiCMOS device, a Bipolar device, or other type of IC device. Those skilled in the art are familiar with the various types of devices which may be located in the IC 800. Illustrated in FIG. 8 are components of the conventional IC 800, including transistors 810, a gate oxide layer 860, and dielectric layers 820, in which interconnect structures 830 are formed (together forming interconnect layers). In the embodiment shown in FIG. 8, the interconnect structures 830 connect the transistors 810 to other areas of the IC 800. Also shown in FIG. 8, are conventionally formed tubs 840, 845, source regions 850, and drain regions 855.

[0066] Of course, use of the detection system of the present invention, or a method employing the system, is not limited to the manufacture of a particular IC device. In fact, the present invention is broad enough to encompass the manufacture of any IC device derived from a substrate that requires at least some degree of polishing along the way. Additionally, although the present invention has been described in detail, referring to several embodiments, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form. 

What is claimed is:
 1. A polishing apparatus, comprising: a polishing platen; and a rotational strain sensor coupled to the polishing platen and configured to detect a change in a rotational strain of the polishing platen during a polishing process.
 2. The polishing apparatus as recited in claim 1 wherein the polishing apparatus further includes a polishing pad coupled to an outer surface of the polishing platen and the rotational strain sensor is a piezoelectric sensor located between the outer surface and the polishing pad.
 3. The polishing apparatus as recited in claim 1 wherein the polishing platen includes a base plate having a backing block projecting therefrom and an upper plate having a cavity formed therein and configured to receive the backing block therein, the rotational strain sensor being located between a face of the backing block and a face of the cavity.
 4. The polishing apparatus as recited in claim 1 wherein the polishing platen includes a backing block projecting from a surface thereof and the polishing apparatus further includes a polishing pad having a cavity formed therein and configured to receive the backing block therein, the rotational strain sensor being located between a face of the backing block and a face of the cavity.
 5. The polishing apparatus as recited in claim 1 wherein the polishing platen includes a base plate and the rotational strain sensor extends from the base plate and the polishing platen further includes an upper plate having a cavity formed therein and configured to receive the rotational strain sensor therein.
 6. The polishing apparatus as recited in claim 5 wherein the rotational strain sensor includes a flexible stud member having adjacent, spaced-apart conductors located within the flexible stud member.
 7. The polishing apparatus as recited in claim 5 wherein the rotational strain sensor includes a rigid stud member supported on a flexible base having adjacent, spaced-apart conductors.
 8. The polishing apparatus as recited in claim 1 wherein the rotational strain sensor extends from a surface of the polishing platen and the polishing platen further includes a polishing pad having a cavity formed therein and configured to receive the rotational strain sensor therein.
 9. The polishing apparatus as recited in claim 8 wherein the rotational strain sensor includes a flexible stud member having adjacent, spaced-apart conductors located within the flexible stud member.
 10. The polishing apparatus as recited in claim 8 wherein the rotational strain sensor includes a rigid stud member supported on a flexible base having adjacent, spaced-apart conductors.
 11. The polishing apparatus as recited in claim 1 further including a motor driven shaft coupled to the polishing platen, the rotational strain sensor coupled to the motor driven shaft.
 12. A method of detecting an endpoint during polishing of a substrate, comprising: pressing a substrate having a first layer composed of a first material and a second layer composed of a second material against a polishing pad coupled to a polishing platen; producing a first rotational strain of the polishing platen by polishing the first layer of the substrate with the polishing pad; producing a second rotational strain of the polishing platen by polishing the second layer of the substrate with the polishing pad; and detecting a change between the first rotational strain and the second rotational strain with a rotational strain sensor coupled to the polishing platen.
 13. The method as recited in claim 12 wherein detecting the change includes detecting a change with a piezoelectric sensor located between the polishing platen and the polishing pad.
 14. The method as recited in claim 12 wherein detecting the change includes detecting a change with a rotational strain sensor located between a face of a backing block projecting from a base plate of the polishing platen and a face of a cavity formed in an upper plate of the polishing platen.
 15. The method as recited in claim 12 wherein detecting the change includes detecting a change with a rotational strain sensor located between a face of a backing block projecting from the polishing platen and a face of a cavity formed in the polishing pad.
 16. The method as recited in claim 12 wherein detecting the change includes detecting a change with a rotational strain sensor that extends from a base plate of the polishing platen and is received in a cavity formed in an upper plate of the polishing platen.
 17. The method as recited in claim 16 wherein detecting the change includes detecting a change with a rotational strain sensor that includes a flexible stud member having adjacent, spaced-apart conductors located within the flexible stud member.
 18. The method as recited in claim 16 wherein detecting the change includes detecting a change with a rotational strain sensor that includes a rigid stud member supported on a flexible base having adjacent, spaced-apart conductors.
 19. The method as recited in claim 12 wherein detecting the change includes detecting a change with a rotational strain sensor that extends from the polishing platen and is received in a cavity formed in the polishing pad.
 20. The method as recited in claim 19 wherein detecting the change includes detecting a change with a rotational strain sensor that includes a flexible stud member having adjacent, spaced-apart conductors located within the flexible stud member.
 21. The method as recited in claim 19 wherein detecting the change includes detecting a change with a rotational strain sensor that includes a rigid stud member supported on a flexible base having adjacent, spaced-apart conductors.
 22. The method as recited in claim 12 wherein detecting the change includes detecting a change with a rotational strain sensor coupled to a motor driven shaft that is coupled to the polishing platen.
 23. The method as recited in claim 12 wherein pressing a substrate includes pressing a substrate located over a semiconductor wafer.
 24. The method as recited in claim 23 wherein pressing a substrate includes pressing a first layer comprising a dielectric and pressing a second layer comprising a metal.
 25. The method as recited in claim 24 further including forming transistors on the semiconductor wafer, forming a plurality of alternating first and second layers over the transistors and interconnecting the transistors to form an operative integrated circuit.
 26. A method of polishing a substrate comprising: detecting a change in a rotational strain during a polishing process of the substrate.
 27. The method as recited in claim 26 further comprising: changing the polishing process when a change in the rotational strain is detected.
 28. The method as recited in claim 26 further comprising: providing a strain gauge to measure the rotational strain. 